Alternating current arc welding equipment



NOV. 10, 1953 Q NEEDHAM ETAL 2,659,036

ALTERNATING CURRENT ARC WELDING EQUIPMENT Filed Aug. 7, 1951 4Sheets-Sheet 1 n l I I l I Hm f H M 4 m n 5/ a +5. 775; T

I c inventors m HZH. 0/112 'Wal J f3, W

- Attorney J. C. NEEDHAM ET AL ALTERNATING CURRENT ARC WELDING EQUIPMENTNov. 10, 1953 4 Sheets-Sheet 2 Filed Aug. 7, 1951 HT-I- 4 I cIWento W@215 mLl/ M.M

Attorney Nov. 10, 1953 J. c. NEEDHAM ET AL 2,659,036

ALTERNATING CURRENT ARC WELDING EQUIPMENT Filed Aug. 7, 1951 4Sheets-Sheet s I I r lventri own mz m 13. 4M

Attorney 1953 J. c. NEEDHAM ET AL 2,659,036

ALTERNATING CURRENT ARC WELDING EQUIPMENT Filed Aug. 7, 1951 4Sheets-Sheet 4 cllwentoz m MI. 0%

Patented Nov. 10, 1953 PATENT OFF ICIE ALTERNATING. CURRENT ARC WELDINGEQUIBMENT' Claims priority; application Great Britain August 11; 1950"15 -Claims.- (Cl. 315-163).

1 This invention relates-=to equipment": f-or-?elec;-: trio arc' weldingand articularly alternat1ng cur= rent arc welding and, in particular; toa simple effectiveapparatus for stabilisingfiand maintai-ming theareused insu'ch welding:

The invention i cant be employed'i in. any, arc welding where' itisdesired tO"1WO1k"With the open-circuit voltage: as: low as possible butit is particularly" usefulinv thea alternating current arc welding" ofmetals such asaluminium'or'its alloys in aniargonatmosphere in whichatreduced open-circuit voltages, there is a tendency torectifyingaction:orfforithe'aarc' to' fail to reigniteat the start' ofthe? half cyclesa of current which": the metal in: question isthecathode. In the process of'welding aluminium in an atmos-' phere" of"argon: or other inert gas; auxiliary means: are. often? employed to'reignite-the are for thefalternate half fcycles inwhich ithevrectifyring" action terrdsz to occur; Onecommon form 01'? such;auxiliarymeansz'consistssof' a: source of high frequencyv sparks?applied across the: arc gap; Such a-high frequeney spark injector isopen'to variousobjections; Thus; it: is liable to cause" interferencewith radio apparatus in the neighbourhood; Consequently, it is: oftenthe practice to switch ofith'e spark: injector oncec-the arc has-beenestablishedjsmas to-avoid undue radio interference but: such procedurenecessitates the i use of a high open-circuitvoltage to maintain :theare; Such ispark injectors o'equire thexuse of high :voltage sourcesandalsoof 1 spark g-apswhich need adjustment and"periodic clean-.-- ing:Timingf the sparks -to= thecommencement of 'therrectifying half cycle:cannot be regulated in typical spark oscillators? to greater: accuracythan the time interval between successive sparks whichis-comnronl-yabout an intervalof the order of a millisecond. Such/adelayof almi-llisecond between-the zercx of current and ithereignitionof the arc is-sufiicien-t to result'in a. marked decrease-in theeffective half cycle of current.

Other known methods oireignitingthe arc depend upon the provision ofelectron valve oscillator uni-ts fol-injecting ahigh frequency voltageinto the arcv and? such apparatus while giving rise to"interference-withneighbouring radio apparatus-can bearrangjed to' restrict such interference to a permitted: band" of freouienoies; Such apparatus; however,is ofnecessity. relatively complicated in design.

The main object of the present: invention is :to provide an improved-auxiliary:apparatus-for reignition of the welding" are v which avoids:most or all of the above drawbacks.

According to the invention; the auxiliary ap paratus consists'of a" lowvoltage source? of power" of: the correct polarity; a switch forconnecting-that sourceto the arc and' a' timing *de=- vice arranged totime the connection of r'thelow power sourcetothearc; The timingdevice-may, forexample, b'eJinitiated-by the rise in voltage across theare gap, a marked'increase in theresistance of the arc, the occurrenceof "zero :current in the: arc and so forth.

Ina: preferred form; the source'of 'powerzis a continuously rechargedcondenser with" an" im'; pedance: for controlling its -discharge; a gasdis; charge tube-is used-ras the switch and'arr electronvalvacontrol'led' by-thevoltageacross=the arc serves as thetiming'devicebycrendering thegas discharge tube conductive; However; other forms ofapparatus may, be employed; according to the invention, for=example,' atransformer-may be used as the'timing device-insteadof the :elec'-- tronvalveror' an electromechanical-switchin stead of thegas discharge tube:

By thepresent invention, a low voltage surge is injected to reignite thearc and such arrangementexhibits the following advantages as com-: paredwith the previous apparatus already referred to.

Radio interference :is absent'and zthereforewthe apparatus "may berkeptin-continuous operation throughout the welding; The welding'trans formermay then be operated-ata substantially lower open-eircu-itvoltage withthe result that there is an increased safety to personnel, lower capitalcosts; improved'power factor and-lower power consumption The invention tenables alu-e minimum andi-its alloys aswell as other'metals to bewelded inan'atmosphere'of argon by the-use of transformers ofinherentlyunsuitable design such as certain leakyreaetance-transformers at anopen-circuit voltage-well Within the-range generally approved.= Thusalternating current argon arcs canbe maintained to alum-iniums'and itsalloys or to copper at an open'circui-t voltage of less than '50 voltsS: The apparatusrm-ay beconnected in any-welding circuit without thenecessity for auxiliary equipmenthouseddnthe welding torch; The design?can be simple and the apparatus canbe easily housed within. a compositewelding'set or provided as an: indee pendent unit" of smalldimensionsforruser existing welding equipment. There arerno' highvoltage sources: Faulty phasing Ol' tl'imlng of the injected? impulsesor surges" cannot occur since the impulse can be: timed 'to occur: ontherecovery voltage transient," thereby; ensuring"? a full half cycle ofwelding current. Surges of either one or both polarities can be derivedas required. No apparatus which requires heavy servicing and maintenanceis employed.

By providing additional circuit connections, the apparatus may bearranged to initiate the welding are across a cold gap without thenecessity of touching the electrode on to the work.

The additional connections required for initiating the arc across a coldgap only need to be be used for a brief time at the start of welding andcan then be switched out, since by the use of the present invention, thearc continues to be reignited once it has been struck. The device forstarting the arc across a cold gap produces only a single spark at eachhalf cycle concerned and therefore, gives rise to less radiointerference than devices in which a number of high frequency sparks areinjected per half cycle.

In carrying out the present invention, a transformer may be interposedbetween the surge injector unit and the welding circuit, and then bychoice of a suitable matching transformer, the equipment can be renderedsuitable for the electrical characteristics of different welding arcsand circuits. In fact, such an intermediate transformer between thesurge unit and the arc affords a number of possibilities of arrangingthe apparatus in accordance with the present invention.

Continuous reversal of the connections by say, a synchronous relayprovides surges of alternate polarities from the same condenser at eachhalf cycle of the power supply. Moreover, if a stepup transformer isused a surge of greater voltage can be applied to the arc. In theapparatus according to the present invention with a simple directconnected surge unit as described above, if the condenser of the surgeunit is charged, say to 300 volts, the arc is subjected to a surge ofonly about 200 volts owing to loss in the discharge tube switch, theseries resistance for controlling the surge and also to the fact thatthe Welding circuit is of low impedance, for example of the order of 1ohm at a frequency of 50 cycles per sceond, but by using a step-uptransformer, the surge voltage applied to the arc can be increased asdesired.

Incidentally, such a high voltage transformer can be used for breakingdown a spark gap so as to apply a high frequency spark to the cold arcgap to start the arc.

A surge transformer employed in accordance with the present inventionmakes it possible to reduce the effect of the open-circuit voltage usedfor welding on the gas-discharge tube used as a used with a weldingtransformer providing unnecessarily high open-circuit voltage because inthe arrangement already described, the timing circuits have toaccommodate a wide range of potentials on the various electrodes of thevalves while self-firing of the switch valve at random must be avoided.

Another possibility in accordance with the invention, is to use atransformer with more than one, and particularly two, secondary windingsbe tween the surge injecting unit and the welding electrodes.

For example, a transformer may be employed with a primary windingconnected to the condenser used as the source through a gas dischargetube or other form of switch, while the transformer has one secondarywinding producing, say a one to one ratio and the other producing arelatively high step-up ratio.

If it be assumed that the secondary winding of the one to one ratio beconnected across the arc electrodes in series with a limiting impedance,and the high tension secondary be connected through a spark gap to thewelding electrodes, a radio frequency choke being provided in theunearthed supply line to prevent a high frequency, high voltage surgewhen the spark gap breaks down from passing back to the source ofsupply, the action takes places in the following sequence.

The voltage across the high tension secondary winding breaks down thespark gap in series with it, and also the cold arc gap and a smallcondenser across that secondary winding very rapidly discharges throughthe arc gap which thus becomes ionized. Then a larger condenser acrossthe other secondary winding discharges into the ionized gap establishingthe arc and maintaining it until it is fully supplied from the powercircuit. Subsequently, when the arc is hot and the gas discharge tubeswitch is tripped, the larger condenser which is charged to the lowervoltage discharges into the hot arc gap before the voltage from thestep-up transformer secondary necessary to break down the spark gap isreached. The charge in the high voltage secondary circuit can also feedinto the hot arc gap through the low voltage winding of the transformer.

The transformer secondary windings can be mounted on a single iron coreor may be pro vided on separate transformers, but as one of thetransformers has a one to one ratio, the arrangement can be simplifiedend yet produce a similar result. Then the larger condenser could beconnected across the primary winding of a high voltage transformer so asto be connected to the welding electrodes through a limiting impedance,and the secondary high tension winding bridged by the smaller condensercan be connected in series with the spark gap to the welding electrodes.In that case the larger condenser in the primary circuit and the smallercondenser in the secondary circuit can operate in the same sequence asthat described above. Similarly the primary winding of the high voltagetransformer may be connected in series with the surge.

In order that the invention may be clearly understood and readilycarried into effect, some examples of welding apparatus according to thepresent invention will now be described with reference to theaccompanying drawings, in which the different figures are circuitdiagrams of the different examples.

In these examples, the connections are such as to produce a positivesurge for reignition of a welding arc in an inert gas at each half cycleof reverse polarity. The connections can, however, be modified toproduce in the welding circuit negative surges or positive and negativesurges alternately.

In Figure l, the welding transformer WT is connected in the usual waythrough a limiting choke L, which may be built into the transformer, tothe are between the aluminium work-piece l to be welded and theelectrode 2, the are being assumed to be formed in an atmosphere ofargon. A condenser 3 having a capacity, for example, of 4 mfds. is showncharged continuously from a source 4 through a charging resistance 5,for example of 2,500 ohms. The source t is illustrated diagrammaticallyas a battery giving a positive awaegoss supply of-say;A50dvo1t tostheupperplaterof the condenser il blifi, of course, mayibe *derived irom a;transformer and rectifier r unit withxon without smoothing-components.Thecondensemaiisi'con nectedsacross or in parallelwoithi the arc i,,2through a: gasdischarge tubef fi and aa resistor- I say, of- 5 ohmst Thedischarge tube fi iswaaecol'd cathodegas -fi1ledtetrode an N: S"; P: Titube 'and serves': as the switch tonthe rsurge in jector.- Other for-Insof "switch may; of cou rse,-;..be used but the advantage of the coldcathode-gas discharge tube over'a mech'anicalsor el'ectro mechanical:switch is that; it is ni'ore-precis' and controlableirntiming because itis' electronic'g and it has-the advantage over a hot cathode'thyratrontubethat there isrno delaydue -to heating run. More usual types ofelectron itu bes'rare?neatloffsufrfici entlyf'low impedance:

The positiveplatevoi the: condenser: 3 1 is::coxi I nected to the anodeai-oi the tube '6 and thetcathe ode c of the: tube. isv connected tother' welding electrode 2, while the negatiye plateofithe-con denser 3'is connecteddi'rectly to theearth l'ine E and! to the work-p-iece--| soas to apply a direct current surgeof "thezcorrect polarity". to thearcto assistin reignitinethe 'latter'at the beginning jof each reversehalf cycle.-

In the particular exampie underconsideration, the screen 'grid sismaintained *ata suitable tentia-lbyiconnection to a potentiometer 8;:say-of 50,000 ohms; through. aw resistance Q of: 10,000 ohms andthecontrol grid gof the tube S' is arranged to have'a negative potentialapplied to it torend'er'the tube conducting, that'is to "say; to

close the switch? and tostart the discharge through the tubeotethe-welding circuit. The control of the grid gc-is Peffecteddby an:electron triode l 0 whose anode "a issupplied from a point ll inthe-di-rect current source affording. aapoe tential'of say300'-volts-:th-1'ough an anoderesistance-lZ-oi say, 30-,000-ohms..-- Theanode'a' is-a-lso connected-to the control grid gyof the'gas. dischargetube nth-rough a condenser 1 3 010101 mid. capacity with aresistan'ce-itot 10,000- ohms con"- nected betweenthe grid gsandfithe earth line E;the-tu-be i0 is controlled from a: tapping :in a p0- tential divider l 5of 50,000 ohms-connected across the welding are l, 2. The controlgrid-g" oi the triode l0-- is-i connected through'a resistor 16a ofl00;000 -ohrnstothe tapping: in the: potential die viderlfiwhichs-isselectedso that when'the arc tendsto become extinguished-thetriode Illa is biased and becomes conductive; therebyt-itsano'depotential is depressed andis: transmitted to. the grid of the-dischargetube- 6 which thereupon becomesconductiveas: already described so thatthe condenser 3 discharges through. the. controllingsresistor l tothe-arcLls The-arc I, 2iis thereby reignited'and is maintainedduringsthe ensuing hali-cycle-by powerfrom the source of welding supplyWT.

In the example shown inFigure- 2 the components are generally the same=and: havevbeen given thesame reference characters- The only differencenow is that thewconnections are-such as toprovide seriessurgeinjection,the negative plate of the condenser 3 being-connected by a conductor i'lto-the' unearthed sideof the sec-'- onda-ry: winding. of thewelding:transformer WT; and the' tri0deilis shown supplied from" ananodenouroo-dkwhioh is independent of the battery'dt- Thereforethepathzof the surgeis-from the-positive plates of' the condenser 3through the gas dischaeg tdbe :s -forming the switch; the control-lingresistor' '13 the are I 2; these'condary 6 windingizot'ithestransfomnerWT; and retuming' bys'the:conductor1"l1 toithe condenser 3: Thus; inthis case, the voltage of the condenser 3 iszaps piiedilto the'ardiitnsfiries! with that not the .secondaryrswinding; ct -the weldingtransformer:WT:

InfFigure 3 ;a system i's shown imwhich tripe ping efiectedi1by?'a1:vo'1tage tra nsformerrVT ins stead .ofby='the tr'iodet I llwandits-circuits which are showninoFigures "1 and =2.- The surge-comnectiorr. is? a parallel? connectionsas s in Figure? 1 but this methodof tripping;- and the othersdzo be describedgucani equally? well-beused; with" the series surger' injector" circuit shown: in Fig-urea 2';Thus; the-"welding: circuit and :the connections of the gas: dischargeswitch tube -6 to the arcvare the' sameras; in Figure- 1 and the; samereference characters: have "been employed:-

math 63156; however, the: secondary winding l8=-=of the fvoltage"transformer VT: is connected acrossi'the "resistance: I awhich;is:-connectedv be.- tween the control: grid 9 'of the gas" dischargetube? 6 and carthxli'ne '13 connected to: the negativerplatepfthescondenser'it." The primary winding; 1913f: the transformerisshown-econnected th'rouglfr-w a condenser 20 across; the: weld?-ingtarcs Thus; as ther potentiai-across-the are i,-, 2,:commencestorise, current flowswthrough the primaryW-inding; i9 rand thie-potential. of -the-'sec-' ondary-wi-nding lads-applied to the gridQ'xOf-bhfi gas, discharge-tube" 6 to: ignite-the latter; In othel respects the system works as -describ'ed :for Figure-1.

The: circuits: shown in. Figure -3 could obviously be slightly, modifiedtov ignite the tubeB byF applying aapiositive: voltage-impulse tothe-screen; grid s of the gas dischargetub'e 6': instead of applya-inegative impulse to the control" grid 1 The only difference: issthat-theappropriateend 0-2 the secondaryywinding: i8-wouldcbe connectedthrough'as condenser such as the condensen 21 to v the. screen grid 1s;-..and the-otherend of the secondarywinding, I8 connected to the-earthliner In theiurther example shown inEigure4, the system is genera11y.-.-the same as in Figures 1 and 3, and -the.-same reference characters havebeen employed; The diiterence is that the gas dis charge tube 8 isconnected: so as: tor-be self tripping-:duet-tothe weldingicircuitpotential on itsecathode-c. Its-1 controligrid gis connected through agridcsresistance: 22 to the earth line E and. thence to the negativeplate of the surge condenser; 3-, and its: screen grid 8' is: connectedthroughaaresistor 9 to aatap' in the potentiatdi vidertms before".: Acondenser 23 cou-ld beiused' connected, (as zshown in :dottedlines;between. the cathode coandthesereen grids. to: pass :theiransient-ivol-tageiot theteathode c direct to the screen grid 3'.-

A slight-v modification.- of the"? arrangement shown in 1 Figure 4 is':illustrated in Figure,- 5, in which; the are circuit-land thesurgeinjector circuitareoasin Figuresfl, dand i'and-ithe same' reference:characters have" been employed. Inv this case,.the:screengrid'potential divider this connectedbetween-the anode a-an'd thecathode c: at Y the: gas discharge" switch-tube v6 therscreen grid as.as before being connected througha resister 9 tothe Ftapping ofitheipotential divider i 8 With-this-- connection; when the" arc is" extin=guisheduand the potential of the cathode c rises;- the'potenti'al:between thecontrol grid g and the screen grid'sis increased and thedischarge tube 6" tripped. A" small condenser 24; say of' 0.01 capacityand similar to the condenser 23 in Figure 4, may be connected betweenthe oath-- ode c and the screen grid s as indicated in dotted lines.

In some cases, it is not desirable to touch the welding electrode 2 onthe work-piece I and then other means may be provided for starting theare. In Figure 6, an arrangement for that purpose is shown applied tothe welding system illustrated in Figure 4. The method of tripping shownin this case is by means of an electron triode ID as in Figures 1 and 2,but in this and examples to be described later, the alternative methodof transformer tripping as shown in Figure 3 or Where suitable,self-tripping as shown in Figures 4 and 5 may be employed. In Figure 6the surge condenser 3 and the gas discharge tube 6 are arranged forstarting the arc to be connected by a switch 24 to the primary winding25 of a high voltage transformer ST, while the secondary winding 26 ofthat transformer is shunted by a condenser 21 of say, 0.01 mfd. andconnected through a spark gap 28 across the supply leads of the weldingelectrodes I, 2. On closing the switch 24 on to its contact 29, thetransformer ST generates a voltage to break down the spark gap 28 andthus to initiate the arc. The resulting high frequency spark isprevented from passing back to the welding transformer WT by inserting aradio-frequency choke coil 30 between the welding transformer WT and thespark gap 28 with a high frequency by-pass condenser 3| connected acrossthe main welding circuit.

Apart from the range afforded by the alternatives already described andby the choice of dimensions, the application of a surge injector unit lcan be increased by introducing a surge transformer between the unit andthe welding circuit proper, and such a transformer may match the surgeset up to the requirements for reignition of difierent arcs. An exampleis illustrated in Fig ure '7 in which such a matching transformer AT isintroduced. The circuit in Figure 7 is similar to that of Figure l inregard to the condenser 3 and its supply circuit and the tripping of theswitch discharge tube 5 by the electron triode I I]. The surgecontrolled by the discharge tube 6 in Figure 7 flows through thecontrolling resistance 1 and the primary winding 32 of the transformerAT which is a step-up transformer which is useful in cases where theinherent voltage of the 13' surge is limited, for example, by thebreak-down potential of the discharge tube 5. Such a transformer mayalso compensate for losses in the switch discharge tube 6 and theimpedance 1,

which result in the voltage applied to the are I,

2 being less than that available at the condenser 3. The secondaryWinding 33 of the transformer AT is, as illustrated used with a seriescondenser 34 of such a capacity that the surge at the step-up voltage isrelatively freely passed into the welding circuit, whereas the voltageof the mains frequency is attenuated by this condenser as far as theinjector unit is concerned. If it is desired to increase the magnitudeand duration of the current fed to the are from the condenser 3, thetransformer AT may be a step-up transformer.

In Figure 8, another example is shown in which the output from thesecondary winding 26 of the transformer ST charges up a small highvoltage condenser 21, say of 0.001 mfd. capacity, in a similar manner tothe condenser 21 in Figure 6, and the surge from the condenser 21 isvery suddenly applied to the are I, 2 by the breakdown of a pulsesharpening spark gap 28. In this way, high voltage sparks may beproduced from a low voltage surge and the voltage of the sparks isdetermined by the setting of the spark gap 28 up to the maximumavailable from the transformer ST and the small condenser 21. The sparkmay be timed in relation to the open circuit voltage or are conditionsto produce the best conditions of initiating the are by the adjustmentof the tripping circuit for the switch tube 6. In Figure 8, it will benoticed that this switch tube is arranged for tripping by the electrontriode I0 as in Figure 1.

The effectiveness of initiating the are by means of a spark as justdescribed is greatly increased if the spark is combined with a surgetaking place at the same time. The surge provides the current passinginto the gap newly ionized by the spark until the power welding circuitcan take over, whereas the latter does not always readily follow up thebreak-down of the arc gap by the spark. The surge should be timed tooccur, say, a few microseconds before the spark since the latter is ofvery short duration.

In Figure 9, a circuit arrangement is shown in which there are twotransformers fed by a single condenser 3 and switch discharge tube 5.The transformer AT produces the surge through the resistance I in itssecondary circuit and the transformer ATI produces the high voltage towhich the small condenser 21 is charged for breaking down the spark gap28 and applying the sparks to the arc circuit. The primary windings ofthe transformers AT and ATI are shown connected in parallel to receivethe discharge current from the condenser 3 but it will be realised thatthey can equally well be connected in series and, in fact, the windingsof the two transformers could be placed on th same core. The low ratiotransformer AT for transmitting the surge may have a condenser, say of 1mid, in parallel with its secondary winding as shown in dotted lines at35, to store some of the input surge energy and to discharge into theare I, 2 when the gap has been ionized by the spark from the transformerATI and condenser 27. The low ratio surge transformer AT may also have acondenser 3 in series with its secondary winding as in Figure 7 to passthe surge but to attenuate the main frequency voltage.

Another small modification is shown in Figure 10, Where the low voltagetransformer AT is not used but the primary winding of the sparkproducing transformer ATI is connected in parallel with the surgeinjector unit comprising the condenser switch discharge tube 6 andcontrolling resistance I, and the welding circuit itself. If required acondenser 35 may be included in parallel with the surge injector unit asshown in dotted lines similar to the condenser 35 in Figure 9.

As a small modification, the primary winding of the spark-producingtransformer ATI may be connected in the surge circuit in series with theresistance I. With a suitable transformer arrangement, the above circuitarrangements for producing a low voltage surge and a spark can provideautomatic change over from spark ignition to surge reignition of thearc. Thus, when the arc is cold or not sufficiently ionized, the surgefails to break down the gap but, a spark is produced which does breakdown the arc gap and starts an arc, and when the arc gap is sufficientlyhot, the low voltage surge can reignite the are before the spark voltageis built up and, in addition, with certain transformer arrangements. theenergy in the spark circuit is then re- "9 turnedtto the surge and 1hence to f the are by transformer action. .Furthermore, it is -possibleto change over from spark initiation to surge-reignition, for.exarnple,:in the circuit show-n in Figur i9, :by switching-out eitherthe transformer AT .ort'he .transformeriATL- by :means for example, 'ofan electro=mechanical relay operated bythe current or voltagein-theavelding circuit.

Finally, in Figure 11 a circuit arrangement=is showniiorprod-ucingacombinedsurge and spark producmgoperation of the apparatus with switchchange-over to surge "reignition of the welding arc operation alone. Th:"primary -windi=ng-f the spa'rk-producing transformer -A'Il --isconnected in series with the surge circuit and may be a'standardmotorcar ignitioncoil, witha high voltage condenser?! :of 0.001 mid. capacityconnected across its secondary winding and connected to the arc gap l, 2through a sparkggap 28-With!the1hig'h' frequency chokefcoil30-;andlbypass condenser 3| to preventthesparkiromfibein applied to themain power transformer WT and. chokecoil L. The prir'nary .windingofethe spark transformer 'ATl'is energised by opening the switch SI. Thesurge injector, as before, comprises a condenser 3, switch dischargetube 6 and controlling resistance 1, and has another condenser 35 ofsay, 4 mid. capacity to store some of the surge energy, connected inparallel with the applied surge between the switch tube 6 and theresistance 1 by closing the switch S2. The surge-storing condenser 35enables the high voltage surge on the condenser 29 to be built up toapply a spark t0 the welding circuit approximately at th instant of thepeak of the applied surge voltage. In addition, the tripping or timingcircuits to the switch discharge tube 6 are altered so as to cause thespark to be applied approximately at the instant of the peak of themains frequency welding circuit voltag when the are gap requires aspark, but so as to cause a surge to be applied as soon as the arccommences to become extinguished where the surge alone is used toreignite the are. This is accomplished in Figure 11 by closing theswitch S3 which connects a condenser 36, say of 0.02 mfd. capacitybetween the grid 9 of the tripping triode l0 and the earth line E todelay the application of the potential to that grid from the potentialdivider I4 and resistance l5.

Alternatively, a resistance of say, 100,000 ohms may be inserted betweenth grid 9' of the tripping triode l0 and the earth line E so as toreduce the applied potential to that grid.

The change-over from surge injection only to a surge combined with aspark for igniting the are when the gap is too highly de-ionized iscarried out by closing the switches S2 and S3 and opening the switch $1.This may readily be automatically carried out by a relay controlled bythe voltage across the arc gap I, 2. Such relay is designed to beenergized by the welding opencircuit voltage, but not to be operated bythe arc voltage.

What we claim is:

1. In an alternating current arc-welding system, the combination of apair of arc-welding electrodes connected to a source of alternatingcurrent, a source of direct current, a normally open circuit extendingbetween said source of direct current and said arc for applying acrosssaid electrodes a unidirectional current pulse of a voltage of less than500 volts, and a switching device responsive to the condition of the arcacross iii sai'd'electrodes for-closing; said current. when the arc.commences to extinguish.

'12.. Arc-welding apparatuszaccordingfitozclaim '1 whereinsaidxzswitching:..cleyice ,cornprisesnarvoltage "responsive devicezope-ratiyeoto ,elose. said air..- rcuitwhen :the voltage'aacross saidelectrodes ex ceeds a predeterminedva-lue.

33. .-.Arcewelding apparatus-according to claim 1 wherein.saidxsourceuof direct currentcomprises a .1condenser,;and 1a ichi'ghIICSlStaDGI-Z circuit tor continuously :gcharging ;said ...condenserfrom :an external-x-sourcebtcurrent.

' 4. Arc-welding:appanatusaaccording. toxclaim 1 wherein isatidswitpching :device comprises .-.a'. nor.- mallysinactive ,gaseons;discharge tube ;.connected tO'zbfi rendered-:active byianzincrease inthevoltage across said electrodes.

5. Arc-welding apparatus according to:.=claim14 wherein isaid'zgaseousdischarge :device isa-provided with adischargegcontrollingselectrode,and including an electron :discharge'tube con-trolled by-the voltage:developedacrossxsaid.welding electrodestor energizingssaid control: electrode.

6. Arc-welding apparatus according to claim 1 wherein said switchingdevice comprises a voltage responsive device, and including atransformer energized by the voltage across said welding electrodes forcontrolling said voltage responsive device.

7. In an alternating current arc-welding system, the combination of apair of arc-welding electrodes connected to a source of alternatingcurrent, a source of direct current, a normally open circuit forconnecting said source of direct current across said welding electrodesto supply direct current thereto, and a switching device responsive tothe operating voltage across said welding electrodes for closing saidcircuit when said operating voltage exceeds a predetermined value.

8. Arc-welding apparatus according to claim 7 wherein said switchingdevice comprises a normally inactive gaseous discharge tube connected inseries with said source of direct current across said weldingelectrodes.

9. Arc-welding apparatus according to claim 8 wherein said gaseousdischarge tube is provided with an anode, a cathode, and a controlelectrode interposed between said anode and cathode, and meanscontrolled by the voltage across said welding electrodes for applying acontrol potential between said control electrode and said cathode.

10. In an alternating current arc-welding system, the combination of apair of arc-welding electrodes connected to a source of alternatingcurrent, a source of direct current maintained at a voltage of less than500 volts, a normally open circuit extending between said source ofdirect current and said are for applying across said electrodes aportion only of the voltage of said source of direct current, and aswitching device responsive to the voltage across said electrodes forclosing said circuit when the arc voltage exceeds a predetermined value.

11. In an alternating current arc-welding system, the combination of apair of arc-welding electrodes connected to a source of alternatingcurrent, a source of direct current, a normally open circuit extendingbetween said source of direct current and said are for applying acrosssaid electrodes a unidirectional current pulse, a switching deviceresponsive to the condition of the arc across said electrodes forclosing said circuit when the arc commences to extinguish, a spark gapand a condenser connected in series across said welding electrodes, astep-up transformer having its secondary winding connected across saidcondenser, and switching means for completing said normally open circuitthrough the primary winding of said transformer whereby a high voltagearc-starting impulse may be applied across said electrodes.

12. An arc-welding apparatus according tq claim 1 wherein said normallyopen circuit is coupled to the welding arc circuit by a transformerwhich matches the surge set up to the conditions occurring in thewelding arc circuit.

13. An arc-welding apparatus according to claim 12, wherein the couplingtransformer is a high voltage, step-up transformer, the secondarywinding of which charges a high voltage condenser which is connected tobreak down a spark gap and thus discharge into the welding arc circuitat the predetermined instants determined by the conditions of the arc.

14. An arc-welding apparatus according to claim 13, wherein a single lowvoltage source connected to pass impulses at timed instants,

feeds the impulses to the primary winding of two transformers one ofwhich provides surges in its secondary circuit which flow through thewelding arc and the other provides in its secondary wind ing the highvoltage for charging the condenser which breaks down the spark gap andapplies the sparks to the welding circuit to ionize the arc gap inreadiness for the surges from the first transformer.

15. An arc-welding apparatus according to claim 14, wherein the windingsof both transformers are mounted on the same core.

JAMES CHRISTOPHER NEEDHAM. LEONARD HERBERT HUMPHREY ORTON.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,982,327 Smith Nov. 27, 1934 2,235,385 Rava Mar. 18, 19412,365,612 White et a1 Dec. 19, 1944 2,482,473 Fromm Sept. 20, 19492,495,183 Roberts Jan. 1'7, 1950

